Chrome or nickel plated copper tube source?

These are quadrafilar Helix antennas. Two spiraled loops about 24" x 16". I definitely agree that soft drawn copper would be delicate. I made the prototypes in hard drawn and it is strong enough for most situations but it will turn green and stain the deck over time. That is why I tried the stainless. The problem ie the resonant frequency has to be pretty exact and the higher resistance throws everything off.

Some 1/4" Beryllium/copper rod would be close to ideal. It is strong, about the same conductivity as copper and ages to a sexy golden brown. Keep an eye out for some 4' to 6' lengths.

Beryllium copper is also an alloy and does have a higher resisitivity then pure copper. Not as high as stainless but definitely higher than copper.

Have you considered fabricating one out of copper, and then putting heat-shrink tubing over the outside of it? YOu can purchase that in long rolls.

Jim

How about epoxy coating a copper antenna? If you do, thorougly clean the copper, prime with Dow-Corning Z6020 diluted with alcohol, add a little Z-6020 to the epoxy and add some mica flakes for UV resistance. I could send you an app note if you wish.

Ted

Not a good idea. The heatshrink will change the resonant length of the elements, ie dielectric shortening. That'll mess up the design dimensions.

Gary

By how much? From the dimensions given, I would guess about 3m wavelength (don't know exactly). A layer of heat shrink would be

The difference is easily calculated. Protecting the outside with a polymer of some sort is the way to go.

Jim

The only trouble with the teflon is you need to get it smokin' hot to shrink it, and it doesn't really shrink as much as the regular stuff.

I would think a thin layer of polyolefin would have a low enough loss for the application, and it can be purchased in roll quantities.

Jim

Now there is an idea. I think I will clean up the best prototype, paint it with linear polyurethane and see how it does.

Heat-shrink polyolefin tubing *is* a polymer! :^)

Jim

Polyolefin should produce a velocity factor of 0.8. So the elements will be about 20% too long.

Gary

That's in a transmission line where the E field is mostly in the dielectric. A thin skin of dielectric on an antenna element won't make much difference because the vast preponderance of the near E-field is still in air.

Plat>Polyolefin should produce a velocity factor of 0.8. So the elements will

Prune to resonance. Easier than having to add some on!

Jim

I think nickel plating would work fine.

See earlier post for site that calculates skin depth. For nickel at 137.5 MHz it's about .0005". Just for fun I calculated the resistance of a 1/2" dia layer of nickle .0005" thick and 6 feet long. It came out to 0.29 ohms. The effect of that on a 50-ohm antenna would be difficult to measure.

You sure don't want to shrink it down over Radio Shack grade insulation but over bare copper it shouldn't be a problem.

True but I have some I picked up at Boeing Surplus that shrinks by about

Probably. But I thought it might be interesting to mention it. I have used it on a number of projects with excellent results.

Ted

Right on.

On real stuff (not silly stuff like speaker cables), the gold plate is very thin and often over silver. A couple of skin depths of silver for conductivity and a fraction of a skin depth for corrosion resistance. BTW, the gold dould be brush plated for (probably) less cost in materials.

Ted

Honestly I've had trouble using it here at work, even with a good 1500 watt heat gun. It just takes forever and the item it's getting shrunk on to gets just about hot enough to melt solder.

I think this is one reason why teflon-insulated wire is invariably silver-plated under the teflon - there must be some very hot temperatures involved that would mess up solder or tin plating on the wire.

But of course, on bare copper, it should work fine.

Jim

You are comparing two different numbers, Don. The 50 ohm number represents the impedance of the antenna, not the actual loss in the conductors.

The loss in a piece of copper tubing like the one we are discussing is probably better off represented by some number measured in terms of a few milliohms. A third of an ohm of resistance might well lower the Q of the circuit enough to cause a problem.

Jim